1. Field of the Invention
The present invention relates to color spatial filtering for ink jet printers or the like and more specifically, to an enhanced print mode and associated circuitry for a thermal ink jet printer which corrects black pixel coloring to increase black picture quality and reduce intercolor bleeding.
2. Description of Related Art
There is a need for high quality black text and graphics and color text graphics and pictorials on plain paper. Current state of the art thermal ink jet printers have fast drying CYMK (Cyan, Yellow, Magenta, Black) ink for color and low intercolor bleed, but the black picture quality needs to be improved (e.g., line edge sharpness).
Various methods of modifying color data prior to printing are known. U.S. Pat. No. 4,959,790 to Morgan discloses an apparatus and method for producing color corrected reproduction of colored original images. This particular apparatus was designed for the purpose of correcting for the effects of linearity failures of various coloring agents. A scanner provides input signals representing primary color readings. A memory stores increments of primary color readings and correlation factors. The apparatus determines final coloring agents based on contribution amounts from the respective coloring agents read from the scanner. This method has a black enhancement mode which merely adds a black color if criteria for a particular pixel indicate a high density at that pixel location. This enhancement is for the purpose of enhancing representation of shadows in the image.
U.S. Pat. No. 4,724,477 to Ellis et al. discloses an image enhancement apparatus wherein an image is represented by color picture components. Fringe signal generators derive a plurality of sharp and unsharp signals in a predetermined manner, and a correction circuit connected in parallel with the fringe generators modifies the signals to provide modified output signals. A black definition signal is also derived from the original pixel signals and summed with the original signal to derive a black output signal. Preferably, the black fringe signal is only summed when the black value is above a predetermined threshold.
U.S. Pat. No. 5,070,413 to Sullivan et al. discloses a halftoning method for creating a continuous tone color image which improves perceived visual color by including a visual color blur function such that a perceived color vector rather than binary color error is propagated. Input color vectors are transferred to a preferred color space to give new color vectors. An output vector is then selected for each new vector color. A neighborhood of previously selected output vectors weighed by a human visual system blur filter produces a blurred output vector. This method is used to correct a continuous tone color image, reducing noise and worms.
Numerous problems are associated with present enhancement methods and apparatus. In particular, no known apparatus addresses a problem inherent with current ink jet printers relating to high quality color image production while reducing or eliminating intercolor bleed. This intercolor bleed is due to the use of a slow drying black ink for four color ink jets. While the slow drying ink commonly used provides high quality reproduction of black images, it also can easily bleed into adjacent areas which may contain other colors. If a fast drying black ink is used for the printer, the intercolor bleed is reduced or eliminated, but accompanying that is a reduced quality of black reproduction since current fast drying black inks have lower picture quality than their slow drying counterparts. Additionally, black could be reproduced by a combination of the other colors, i.e., Cyan, Yellow and Magenta (CYM). This too reduces intercolor bleed, but also results in a reduced quality of black reproduction since current colored inks CYM do not provide the high quality picture associated with printing with a good black ink.
Accordingly, there is a need for an ink jet printer which is capable of reproducing black on a pixel- by-pixel need based on characteristics of surrounding pixels such that the prior compromise between intercolor bleed and high quality need not be sacrificed.
Future ink formulations and papers may improve drying time and reduce spread, and even black picture quality, but present printers need to resolve these problems without unduly increasing printer costs.